On Mac, QTabWidget with setDocumentMode(true) should be visually "unified" with the titlebar.

Indeed, that is the very intention of the call to setDocumentMode. See this Qt Labs blog post (particularly the screenshot): http://labs.trolltech.com/blogs/2008/07/02/some-qtabbar-qtabwidget-love/

Unfortunately, the tabs and window titlebar do not get "unified". I've tried the latest 4.5 and 4.6 versions, using both Carbon and Cocoa. Here is what it actually looks like: http://boxybutgood.com/~jeff/QTabWidget.png

The problem happens both when you programatically set a QTabWidget as the QMainWindow's central widget and also when using Qt Designer. Interesting to note: when using Qt Designer, the PREVIEW mode displays the desired behavior! But when you actually use the class in your application, you get the non-unified appearance.

Also, fwiw, setUnifiedTitleAndToolBarOnMac(true) has no effect on this.